EXAMINATION OF MUSCLE QUALITY AND MOTOR UNIT BEHAVIOR OF THE FIRST DORSAL INTEROSSEOUS OF NORMAL AND OVERWEIGHT CHILDREN

Abstract

It is unknown whether differences in muscle quality or motor unit (MU) behavior exist between normal weight (NW) and overweight (OW) children Purpose: Therefore, the purposes of this study were to examine potential differences in the first dorsal interosseous (FDI) between NW and OW for peak maximal voluntary contraction force (MVC force), muscle cross-sectional area (CSA), subcutaneous fat (sFAT), and echo intensity (EI), as well as parameters of motor unit behavior, specifically motor unit action potential size (MUAPSIZE) vs. recruitment threshold (RT), mean firing rate (MFR) vs. MUAPSIZE, and MFR vs. RT relationships, as well as EMG amplitude (EMGRMS[normalized as %max]). Methods: Anthropometric assessments were taken for body mass, BMI, and percent body fat (%BF). Ultrasonography scans of the FDI were completed in order to determine the muscle CSA, sFAT, and EI. MU behavior will be assessed during isometric muscle actions of the FDI at 20% and 50% MVC by the decomposition of the EMG signal from the surface of the skin. Results: OW was significantly greater than NW for mass (P < 0.001, NW = 30.96 ± 3.68 kg, OW = 41.37 ± 7.31 kg), BMI (P < 0.001, NW = 15.96 ± 0.94 kg/m2, OW = 21.22 ± 2.19 kg/m2), %BF (P < 0.001, NW = 17.01 ± 3.25%, OW = 31.01 ± 4.97%), sFAT (P < 0.001, NW = 2.19 ± 0.60 mm, OW = 3.71 ± 0.97 mm), and EI (P = 0.002, NW = 47.99 ± 6.01 AU, OW = 58.90 ± 10.63 AU). There were no differences between groups for CSA (P = 0.688, NW = 1.138 ± 0.146 cm2, OW = 1.162 ± 0.156 cm2), MVC force (P = 0.790, NW = 14.81 ± 3.49 N, OW = 14.43 ± 3.87 N) or MVC force/CSA (P = 0.697, NW = 13.00 ± 2.84 N/cm2, OW = 12.52 ± 3.60 N/cm2). However, NW had significantly greater lean CSA (CSA/EI) than OW (P = 0.040, NW = 0.024 ± 0.004 cm2/AU, OW = 0.020 ± .005 cm2/AU). For the MUAPSIZE vs. RT relationships, A terms were greater for NW (0.185 ± 0.12 mV) than OW (0.091 ± 0.05 mV) (P = 0.002), and for the MFR vs. MUAPSIZE relationships the B terms were less negative (P = 0.039) for the NW (-1.98 ± 1.36 pps/mV) than OW (-2.79 ± 1.46 pps/mV). There were no differences between groups for the slopes and y-intercepts from the MFR vs. RT relationships or for EMGRMS, however, slopes for the 50% MVC were less negative (-0.646 ± 0.18 pps/%MVC) than the slopes for the 20% MVC (-1.46 ± 0.64 pps/%MVC) and EMGRMS was greater for the 50% MVC (86.23 ± 37.7%) than the 20% MVC (32.27 ± 12.4%). Discussion: It is plausible that alterations in muscle architecture and/or stiffness as a result of greater intramuscular fat allowed the OW group to produce similar isometric MVC strength to the NW while possessing less lean CSA. This is supported by the finding that NW showed greater MUAPSIZES than OW at similar RTs and MFRs, which can be an indication of greater motor unit and muscle fiber size. The MFR vs. RT relationships for both groups agreed with the onion skin scheme, and were similar to what has previously been reported in adults, however, EMGRMS for the 50% MVC was greater for children in the current study than what has been previously reported for adults or aged individuals during similar contractions of the FDI.